1. Field of the Invention
The present invention relates to a method for fabricating a set of distributed feedback (DFB) semiconductor laser devices. More particularly, the present invention relates to a method for fabricating on a single wafer a plurality of DFB semiconductor laser devices, designed to emit laser beams having desired wavelengths different from one another by short step differences. In addition, the present invention relates to a method for fabricating a set of DFB semiconductor lasers best suited for light sources for use in a wavelength division multiplexing scheme in optical transmission systems.
2. Description of the Related Art
A wavelength division multiplexing (WDM) optical transmission system transmits optical signals having a plurality of different wavelengths via a single optical fiber to greatly expand the capacity of the optical transmission system. As the light source for use in the optical transmission system, a DFB semiconductor laser device is generally employed due to the excellent wavelength selectivity thereof.
The WDM optical transmission system mainly employs wavelength bands in the ranges of 1530 nm to 1565 nm (C-band wavelengths) and 1565 nm to 1625 nm (L-band wavelengths). Each band requires a set of DFB lasers that provide a lineup of a few tens of wavelengths different from one another by a 100 GHz (about 0.8 nm) or 50 GHz (about 0.4 nm) step wavelength in accordance with the ITU Grid.
The lasing wavelength xcexDFB of a DFB laser device can be set independently of the peak gain wavelength of the active layer, which corresponds to the photoluminescence wavelength xcexPL, and thus the peak gain wavelength is assumed the same as xcexPL hereinafter. Assuming that xcex9 is the spatial period of the diffraction grating and neff is the effective refractive index of the waveguide, the lasing wavelength xcexDFB is expressed by
xcexDFB=2xcex9neff.
The difference xcex94xcex between the peak gain wavelength xcexPL and the lasing wavelength xcexDFB should fall within a certain range to provide an excellent lasing property of the DFB laser device. The difference xcex94xcex (=xcexDFBxe2x88x92xcexPL) is generally called amount of detuning or xe2x80x9cdetuning amountxe2x80x9d.
The optimum detuning amount depends on the purpose. For example, the desired detuning amount for lasing at a lower threshold current is preferably xe2x88x9210 nm to +10 nm. On the other hand, to provide a higher-speed operation and achieve a reduced spectrum width, the detuning amount is preferably xe2x88x9220 nm to 0 nm, whereas the detuning amount is preferably 0 nm to +20 nm to provide an improved operation at high temperatures and a higher optical output.
In this context, extensive studies are conducted on the method for fabricating a plurality of DFB lasers on a single wafer at a time, in which the DFB laser devices satisfy the predetermined detuning condition and provide the lineups of C-band and L-band wavelengths.
One of the methods is described in JP Patent Laid-Open Publication No. 2000-101187. FIG. 1 is a schematic sectional view illustrating the main configuration of the DFB laser devices described therein, wherein diffraction gratings 24 are first formed on an InP substrate 12 of a 2-inch wafer, to have respective periods which increase along with the radial position of the wafer and concentrically with the wafer. Then, by using a semiconductor epitaxial growth system, active layers 16 of strained InGaAsP multi-quantum well (MQW) structures having bandgap wavelengths that satisfy the above detuning condition are formed in accordance with the lasing wavelength distribution, the lasing wavelengths being generally determined by the periods of the diffraction gratings 24. Thus, the DFB laser devices having the structure as shown in FIG. 1 are fabricated. As shown in FIG. 1, the diffraction grating 24 is formed in the vicinity of the InP substrate 12 with respect to the active layer 16.
It is recited in the publication that the above arrangement makes it possible to efficiently fabricate a plurality of DFB laser devices on a single wafer, each of the DFB laser devices having a unique emission wavelength.
In the method described in the publication, the diffraction gratings 24 each having a unique period are first formed within the surface of a wafer. Then, the active layers 16 having bandgap wavelengths that satisfy the specified detuning condition is formed in accordance with the lasing wavelength distribution of the diffraction gratings 24. The method described in the publication, however, involves the following problems.
First, in the current epitaxial growth techniques, it is practically difficult to fabricate active layers having a specified composition, with a suitable reproducibility in the mass production. This in turn makes it difficult to allow the detuning amounts of the DFB lasers to fall within the range of the specified detuning amount.
Second, when the active layers formed do not provide the predetermined distribution of bandgap wavelengths, the detuning amounts are to be different from the predetermined detuning amount, because the diffraction gratings are fabricated prior to the active layers. As a result, it is necessary to iterate the fabrication process of the DFB laser devices from the beginning, including the formation of the active layer and the diffraction gratings. This makes it difficult to improve the productivity of the DFB laser devices.
It is therefore an object of the present invention to provide a method for fabricating a plurality of DFB laser devices on a single wafer at a time with ease, in which each DFB laser device satisfies the specified detuning condition and emits a laser beam at a unique wavelength.
The present invention provides a method for fabricating a plurality of DFB semiconductor laser devices on a single wafer at a time. Each of the DFB semiconductor laser devices emits a laser beam having a unique wavelength which differs from another wavelength by a certain step. The method includes the steps of measuring peak gain wavelengths of the active layers within the wafer surface, and forming diffraction gratings based on the measured distribution of the peak gain wavelengths of the active layers within the wafer surface, the diffraction gratings having specified periods that allow the detuning amounts of the active layers to fall within a specified range.
According to the method of the present invention, in-plane variances in the bandgap wavelengths of the active layers can be evaluated in advance prior to the formation of the diffraction gratings. Thus, in consideration of the results of the evaluation, the periods of the diffraction gratings can be designed, thereby making it possible to control the lasing frequencies with a high degree of reproducibility within the wafer surface.
In accordance with the method of the present invention, since the detuning amount is controlled to fall within the specified range across the wafer surface, it is possible to provide a plurality of DFB laser devices which are fabricated uniformly within the wafer surface and operative at a lower threshold current and a higher efficiency.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.